A surface acoustic wave device, which is an electromechanical transducer device utilizing surface waves propagated on the surface of an elastic body, has a general structure as shown in FIG. 1, for example. In such a surface acoustic wave device 60, a piezoelectric phenomenon which is caused by a piezoelectric member 61 is utilized for exciting surface waves. When an electric signal is applied to interdigital electrodes 62 which are provided on the piezoelectric member 61, the piezoelectric member 61 is distorted to generate surface acoustic waves, which in turn are propagated along the piezoelectric member 61 and received in other interdigital electrodes 63 as an electric signal. Assuming that .lambda..sub.0 represents the electrode period in the interdigital electrodes 62 and 63 and v represents the velocity of the surface acoustic waves in relation to frequency characteristics, this device has a bandpass characteristic with a center frequency f.sub.0, which is expressed as f.sub.0 =v/.lambda..sub.0.
The surface acoustic wave device can be miniaturized with a small number of components, while it is possible to input/output signals on a surface wave propagation path. Such a device can be used as a filter, a delay line, an oscillator, a resonator, a convolver, a correlator etc. In particular, a surface acoustic wave filter has been practically used by an intermediate frequency filter in television sets for a long time, while the same is now being used as filters for VTR and various communication devices.
A surface acoustic wave device employable in a higher frequency range would be desirable as a surface acoustic wave filter for use in the field of mobile communication, for example. As understood from the above equation, the device has a higher center frequency f.sub.0 in its frequency characteristics if the electrode period .lambda..sub.0 is reduced or if the velocity v of the surface waves is increased. In relation to this, Japanese Patent Laying-Open No. 64-62911 (1989) in the name of the assignee of the present application, for example, discloses a surface acoustic wave device comprising a piezoelectric member which is stacked on a diamond layer for increasing the velocity v of the surface acoustic waves. Diamond has the highest sound velocity among currently known substances, and is physically and chemically stable. Thus, diamond is suitably employed as a material for increasing the center frequency f.sub.0 of the device. On the other hand, it is possible to improve the reliability of the device by increasing the velocity v in the aforementioned manner while relatively increasing the electrode spacing .lambda..sub.0.
A diamond layer can be formed by polishing single-crystalline diamond which is prepared by extra-high pressure synthesis. A diamond layer can also be formed by polishing the surface of a diamond film grown by CVD, for example. Such a polishing step which is necessary for forming the diamond layer, however, requires a considerably long time for working, since it is difficult to work diamond.